The present invention relates to a drive circuit for an organic electroluminescence (EL) apparatus, and more particularly to a color organic EL display apparatus so as to achieve low power consumption.
An electroluminescence (EL) display apparatus is a type of thin display apparatus, in which a thin light-emitting film is applied to a glass substrate, wherein a high voltage is applied via a transparent electrode so as to cause emission of light. Being self-emitting and featuring superior readability and response speed, the EL display ranks with LCD displays in terms of expectations for the future. However, there is a remaining problem with EL displays in achieving low cost, and EL elements are used at present in backlighting applications for such devices as LCD displays, making use of their characteristic of emitting a bright light with a low power consumption.
One disclosed example with regard to a drive method for use in the case in which an organic EL element is used as a backlight for an LCD display or the like, is that in Japanese Unexamined Patent Publication (KOKAI) No.8-211832. In this disclosure, although there is high light-emitting efficiency and an organic EL element at a low voltage, as long as this is used as a backlight, the amount of power consumed is large. Because of this, in the subject disclosure, the configuration is one in which an organic EL element capable of a pattern display is applied, the same image being caused to be displayed on both the LCD element part and the organic EL display element part, the drive electrode pattern being substantially the same, so as to make use of both low power consumption and high light-emitting efficiency.
In this disclosure, the construction is such that the pixels of the organic EL display element part are cause to coincide with the pixels of the LCD display element part, there being a lamination of a substrate, a transparent electrode serving as an anode, a hole injection layer, an organic EL emitting layer and an electrode which serves as both a cathode and a reflector ,from the bottom surface of the polarizer of the upper LCD display element part. In this disclosure, a circuit shown in FIG. 8 is used as a drive circuit for the organic EL display element part.
As shown in FIG. 8, the drive circuit in the color display of an organic EL of the past, supplies current to the elements for each color from a single power supply line, regardless of the color of the light emitted. In FIG. 8, the configuration is one that has the current control section 11 for Blue pixels, the current control section 12 for Green pixels, and the current control section 13 for Red pixels of the color organic EL display, the current drive circuits 31, 32, and 33 which control the current values of the control signal Ib,Ig,Ir, respectively, organic EL elements 41(B), 42(G), and 43(R) serving as backlights and driven by the current drive circuits 31 to 33, and a DC power supply Vdd 51.
A constant voltage is supplied to the organic EL elements 41(B), 42(G), 43(R) from the DC power supply Vdd 51, and the current drive circuits 31, 32, and 33, the current values thereof are controlled in response to an image signal, cause light emission from the organic EL elements 41, 42, and 43. Therefore, in addition to causing emission of light from the organic EL elements 41(B), 42(G), and 43 (R) with a different current value for each of the current drive circuits 31 to 33, each of the same pixels are simultaneously displayed as well on the LCD element parts (not shown in the drawing), so that it is possible to attain a display with both low power consumption and superior read ability.
However, the same voltage is applied to each current drive circuit. Because of this, in an organic EL having voltage versus intensity characteristics as shown in FIG. 2, for a drive circuit of a green (G) element that can be driven with a small applied voltage, the voltage difference between the voltage required for drive and the power supply voltage does not contribute to emission of light. On the other hand, a drive circuit for a blue (B) element requires high voltage, the high voltage is applied.
Therefore, in a drive circuit for an organic EL element of the past, it is necessary to set the applied voltage to suit the element for the color that requires the largest voltage, so that in a drive circuit for an element achieving the required intensity at a low voltage, there was the problem that the power consumption of the drive circuit was large.
Accordingly, it is an object of the present invention to achieve low power consumption in an organic EL color display apparatus, and to further improve the overall drive efficiency.
To achieve the above-noted object, the present invention has the following basic technical constitution.
Specially, the first aspect of the present invention is a drive circuit for an organic EL apparatus having a plurality of organic EL elements performing multicolor light emission, comprising a plurality of DC power supply circuits for each one of colors of emitted lights from the respective organic EL elements, wherein a voltage from one of the DC power supply circuits is applied to one of the organic EL elements emitting one of colors of lights, which is different from the voltage, applied to other organic EL elements emitting respective colored light therefrom.
In the second aspect of the present invention, the DC power supply circuit is a DC-DC converter, and a current drive circuit connected to an output of the DC-DC converter so as to drive the EL elements, is provided, and this current drive circuit is controlled by a control signal responsive to a color signal obtained from an image to be displayed, and drives the organic EL elements.
In the third aspect of the present invention, a power supply voltage controller controlling the DC-DC converter in accordance with a differential voltage between an input voltage of the current drive circuit and an output voltage of the current drive circuit, is provided.
In the fourth aspect of the present invention, the current drive circuit is controlled by a current control section, this current control section having a first circuit detecting differential potential between an output voltage of the DC-DC converter and an output voltage of the current drive circuit, and a second circuit outputting control signal to the current drive circuit in accordance with a voltage detected by the first circuit.
In the fifth aspect of the present invention, the current drive circuit is controlled by a current control section, this current control section comprising a first circuit detecting differential potential between an output voltage of the DC-DC converter and an output voltage of the current drive circuit, and a second circuit comparing the detected differential potential by the first circuit with a reference voltage, and a third circuit outputting control signal to the current drive circuit based on comparison result of the second circuit.